There are many situations in which it is desirable to prove that a still or video image has been produced by a particular image sensor. Moreover, with the widespread availability of digital imaging technology, it may be necessary to prove that the image in question is original and has not been digitally manipulated after being captured.
There are a number of prior art proposals for improving the security and authenticity of images, some of which are based on the use of encryption keys, such as those disclosed in U.S. Pat. Nos. 5,499,294; 5,799,082; and 5,828,751, for example. In U.S. Pat. No. 5,499,294, a process is disclosed in which a unique key is calculated for each sensor and this key is stored in the sensor during manufacture in a specially provided ROM. Such methods thus require additional circuitry that is programmable, and correspondingly require a programming step to be carried out during manufacture, both of which may result in increased costs.